To develop and optimize a layered nanostructure, for example, a biologic specimen, an environmental sample, a semiconductor structure, etc., a scanning electron microscope with a resolution of 1 nm and below is commonly used to scan the nanostructure to obtain a characterization of, for example, chemical elements of the target compositions, pollutants, or defects. By way of example, the nanostructure may be a semiconductor replacement gate structure, as depicted in FIG. 1A, that includes different layers 101 such as a high k dielectric, a work function layer, barriers and metal gate electrodes deposited on a silicon (Si) substrate 102. The nanostructure can be examined in the scanning transmission electron microscope (STEM) by fast two-dimensional scanning of the electron probe and registering the signal from an annular dark-field (ADF) or bright-field (BF) detector. Such fast ADF/BF scanning provides an image of the nanostructure revealing the general morphology of the layers as seen in FIG. 1B. However, it fails to provide specific chemical information, i.e., chemical maps.
Slow-rate STEM scanning in conjunction with an energy-dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS) can examine the distribution of chemicals in the nanostructure as depicted in FIG. 1C. For example, with the transmission illumination scheme of the STEM, the electron probe passing through a sufficiently thin specimen generates the characteristic X-ray radiation, which can be captured by the EDS detector and used for chemical analysis. The EDS signal can be obtained simultaneously with the ADF/BF signal, allowing direct correlation of image and chemical data. However, EDS or EELS spectroscopy requires much longer acquisition from each position of the electron probe as compared with simple ADF/BF STEM scanning. Thus, the spectra are usually taken by scanning not over the entire surface of the nanostructure but rather along one line or a few lines, with further extraction of the chemical profiles along a few directions, as depicted in FIG. 1C. Such chemical profiles are typically noisy and fail to reveal the two-dimensional distribution of chemicals in the nanostructure. Attempts to produce two-dimensional chemical maps have been confronted with the above-mentioned intrinsic slow rate of spectroscopy scanning. For example, the spectroscopic scanning of a frame of 100×100 pixels may take a few hours per frame (as opposed to one second per frame by fast ADF/BF scanning) to achieve a desirable signal-to-noise ratio (e.g., 5 to 10).
To accelerate two-dimensional chemical mapping, dedicated instruments for fast spectroscopy have been introduced. However, they apply a very high electron current (e.g., 0.5 to 1.5 nanoamps (nA)) into the small investigated area, which may incur in-situ damage to the sample and degrade the resolution of the imaging. As a result, the signal-to-noise ratio is often compromised with the resolution in such chemical maps. In addition, the dedicated fast spectrometers are expensive.
A need therefore exists for methodology to accelerate two-dimensional chemical imaging of nanostructures using existing STEM spectrometers with a low electron current while maintaining a desirable signal-to-noise ratio and resolution.